This invention relates to a lighting system, and to lighting fixtures incorporating novel lenses for use in the lighting system. The invention is particularly, but not exclusively, concerned with a dropped pyramidal lens used with a high intensity light source such as a high intensity discharge (HID) lamp for providing improved illumination of building interiors. The invention is also usable with other light sources and may also be used for lighting outdoor areas.
In lighting interior areas, a major consideration is the producing of required lighting levels using as little energy as possible and as little lighting equipment as possible. Thus, the use of light sources having a high efficacy, in terms of lumens per watt, is very desirable. For this reason, fluorescent lamps have become widely used in place of incandescent lamps. Fluorescent lamps, however, have two disadvantages as compared with HID lamps. First, while their efficiency is high, it is not so high as many available HID lamps. Second, fluorescent lighting fixtures are bulky and because of the low lumen output of the lamps, many fixtures are required to light an area to commonly required illumination levels.
The use of HID lamps, therefore, is to be preferred, but two disadvantages of these lamps must be overcome before their widespread use becomes adopted. The first problem is that of glare. Because of the very large amount of light emitted from a relatively small area, the amount of glare can be extremely high if not controlled by proper optical means. The second problem is that because of the very high output, only a small number of luminaires are required in order to produce normal footcandle levels, and thus these luminaires will be spread far apart. Wide spacing of luminaires tends to produce large areas of darkness between the luminaires, with very high footcandle levels beneath them. The light emitted must therefore be spread to give a broad coverage on the working surface, and yet not be so broad that concentrated light is emitted close to the horizontal, causing glare. Presently known HID lighting fixtures do not adequately resolve these problems and do not provide acceptable indoor lighting.